The USS LST-821, renamed the USS Harnett County (LST-821), USS Harnett County (AGP-821), My Tho HQ-800 (Vietnam) and finally LT 57 Sierra Madre (Philippine Navy) was an LST-542-class tank landing ship built for the United States Navy during World War II. She was named for Harnett County, North Carolina and was the only U.S. Naval vessel to bear the name. She served the US Navy in World War II and the Vietnam War. She was transferred to South Vietnam's Republic of Vietnam Navy, which named her RVNS My Tho (HQ-800).
After the Vietnam War, Harnett County was transferred to the Philippine Navy, which named her BRP Sierra Madre (LT-57). In 1999 the Philippine government deliberately had her run aground on Second Thomas Shoal in the Spratly Islands to serve as an outpost of the Philippine Marine Corps to assert Philippine sovereignty in the country's dispute with China over the ownership of the Spratly Islands. She still serves that function.
